what would happen if the battery is selected to off during flight? will the engine shut down? i was told that, if the aircraft is on ground, and the battery is selected to off position when the engine is already running, the engine will shut down. but in flight, the engine will not shut down even though the battery is switched off. is that true? i'm refering to the B737-400.

the engines will not shut down as long as fuel is supplied. This is a complex question as there are a number of systems that are involved in a B737. Once a jet engine is running it no longer needs electrical power but that is on the condition that fuel is still supplied via a mechanical fuel pump via the fuel control unit. Most modern airliners have computers wihich control everything. It is probably likely that the computer would not let the battery be switched out of circuit in flight anyway. I have never worked on a 737 only 727s. Now if you were talking about a light aircraft like a Cessna then power would still be supplied via the alternator, but it is not good for the system to let that run isolated from the battery. A large ripple content would be very bad for the radios etc. The engine would still run as the spark is suupplied via magnetos and the fuel is supplied via a mechanical fuel pump. All engine controls are mechanical. I hope this helps.

In the royal Air Force in the fifties(That dates me!) I first worked on Avro Lincolns (A 126 feet wing span Lancaster, it was designed by the same man) To change the batteries was 2 hours of intense labour. They were installed in front of the main spar and after disconnecting, they were taken one at a time, over the main spar, over the secondary spar, under the mid upper turret, mind your head! Down a 40 inch drop, thats the end of the bomb bay and to the main door. The six feet down to the ground, do this four times and then the same amount of labour with the new batteries. Electricians working on Lincolns develope long arms!